PREDATION BY VERTEBRATES AND INVERTEBRATES ON THE SEEDS OF 5 CANOPY TREE SPECIES OF AN AMAZONIAN FOREST

We studied the pre-germination loss of seeds to invertebrate and vertebrate seed predators of 5 species of Amazonian trees (Astrocaryum macrocalyx - Palmae; Bertholletia excelsa - Lecithydaceae; Calatola venezuelana - Icacinaceae; Dipteryx micrantha - Leguminosae (Papilionoidae); Hymenaea courbaril -Leguminosae (Caesalpinoidae)). These five species were selected from a large tree flora on several criteria. All possess large (3-10 cm) well-protected seeds that might plausibly be attractive to mammalian seed predators. The reproductive biology of three of the species, or close congeners, had been studied elsewhere in the Neotropics (Astrocaryum, Dipteryx, Hymenaea); one is important to the economy of southeastern Peru (Bertholletia); and one, despite large and apparently edible seeds, appeared to suffer no pre-germination loss to predators (Calatola). We conducted the research in mature forests in the Manu National Park of southeastern Peru where mammal densities are unperturbed by human activities. Densities of adult trees of the five species in our area range from very high (> 30 per ha: Astrocaryum) to very low (much less than 1 per ha: Hymenaea). Loss of seeds to all causes, and to mammalian seed predators in particular, was determined for seeds placed in 2-square meter mammal exclosures and in open controls located at 10 m (near) and 50 m (far) from a large mature individual of the target species (with minor variations in the design for Astrocaryum and Calatola). The exclosures were of two types: impermeable - designed to exclude all mammals, but not invertebrate seed predators, and semipermeable - designed to admit small (< 500 g), but not large mammals. Experimental and control plots were stocked with apparently viable seeds during the dry-wet transition period (October-November) and scored one year later. A significant distance effect (higher predation near vs far from a large conspecific adult) was found in only one of the species (Astrocaryum), the only one to be attacked with high frequency by invertebrate seed predators. The absence of any detectable distance effect attributable to mammals suggests that mammals, over the course of a year, thoroughly search the forest floor for seeds. Invertebrates may thus be responsible for most pre-germination distance (density) effects. With respect to the treatments, we found three qualitatively distinct results: seeds of three species (Astrocaryum, Bertholletia, Dipteryx) were significantly protected by the impermeable, but not semipermeable exclosures, implicating small mammals in seed loss; the seeds of one species (Hymenaea) were significantly protected by exclosures of both types, implicating large mammals; and the seeds of one species (Calatola) exhibited 100% survival, whether or not protected by exclosures. The importance of large mammals as seed predators is generally underestimated in these experiments because semipermeable exclosures may serve as foraging reserves for small mammals. Finally, we noted no relationship between the intensity of mammalian seed predation (as suggested by the survival of unprotected seeds) and the abundance of adults of the five species in the environment. The diversity of results obtained for the five species reveals that large-seeded tropical trees may display a wide range of demographic patterns, and points to the likely importance of post-germination bottlenecks in the population biology of many species, even those that may experience severe pre-germination seed loss.